1. Field of the Invention
The present invention relates to an internal combustion engine misfire sensing circuit for sensing misfire by sensing an ion current in the combustion chamber of an internal combustion engine.
2. Description of the Related Art
When combustion is carried out in the combustion chamber of an internal combustion engine, the molecules of a mixture of air and fuel in the combustion chamber are ionized during combustion. When a voltage is applied to the combustion chamber in the ionized state through ignition plugs, a fine current called an "ion current" flows. Since the ion current is made very small when misfire occurs, the occurrence of the misfire can be determined by sensing this ion current.
As shown in the specification of Japanese Patent Application No. 6-8880 as an earlier application which is not yet published (filing date: Jan. 28, 1994), an example of an internal combustion engine misfire sensing circuit is arranged such that an ion current is converted into a voltage, and when the converted voltage exceeds a predetermined threshold value, it is determined that ignition is carried out, whereas when the voltage does not exceed the threshold value, it is determined that a misfire occurred and a two-value signal corresponding to the determination is output.
The known internal combustion engine misfire sensing circuit will be described with reference to FIG. 5, FIG. 6 and FIG. 7. FIG. 5 is a view showing an arrangement of a known internal combustion engine, FIG. 6 is a diagram showing an arrangement of a known internal combustion engine misfire sensing circuit, and FIG. 7 is a timing chart showing the operation of the known internal combustion engine misfire sensing circuit. Note, FIG. 7 shows the respective signals of two internal combustion engine misfire sensing circuits.
In FIG. 5, a known eight-cylinder (#1-#8) internal combustion engine includes ignition coils 2 (2a-2h), ignition plugs 3 (3a-3h) connected to the secondary negative poles of the ignition coils 2 and disposed in the combustion chamber, a power supply 4 connected to the positive poles of the primary coils of the ignition coils 2 and current switching transistors 5 (5a-5h) having collectors connected to the negative poles of the primary coils.
In FIG. 5, each of the transistors 5 has an emitter connected to the ground and a base connected to a combustion controller (not shown). Note, a known internal combustion engine misfire sensing circuit 1a is connected to the ignition coils 2a, 2c, 2e and 2g of cylinders #1, #3, #5 and #7 and a known internal combustion engine misfire sensing circuit 1b is connected to the ignition coils 2b, 2d, 2f and 2h of cylinders #2, #4, #6 and #8.
In FIG. 6, each of the internal combustion engine misfire sensing circuits 1 (1a, 1b) is composed of an ion current sensing circuit 7 for imposing a positive polar voltage on the ignition plugs 3 in the combustion chamber 6 and sensing a negative polar ion current produced by combustion, a current/voltage conversion circuit 8 for converting the negative polar ion current into a positive polar voltage and a waveform shaping circuit 9 for shaping the waveform of an output from the current/voltage conversion circuit 8.
As shown in FIG. 7, when ignition is effected in the internal combustion engine, each transistor 5 is abruptly switched from an ON state to an OFF state in response to a control signal S from the combustion controller. A primary current in each ignition coil 2 is abruptly reduced at the time and a high voltage is generated by a back electromotive force of each ignition coil 2. A voltage generated on the primary coil of each ignition coil is boosted on the secondary coil thereof in accordance with the ratio of windings of the secondary coil to those of the primary coil and appears to the secondary coil of each ignition coil. As a result, a high voltage S2 of about -30 kV is imposed on the ignition plugs 3 as shown in FIG. 7. Note, FIG. 7 shows the signals S1, S2 of cylinders #1, #3 connected to the internal combustion engine misfire sensing circuit 1a and the signals S1, S2 of cylinders #2, #4 connected to the internal combustion engine misfire sensing circuit 1b and omits the signals of the other cylinders.
The ion current sensing circuit 7 accumulates an electric charge in a capacitor 11 which is sufficient to sense an ion current, making use of energy obtained at the time of ignition and senses the ion current by a voltage supplied from the capacitor 11 immediately after the occurrence of the ignition. A current, at the time of the ignition, flows in the direction of arrow 3a in FIG. 6, causes discharging at the ignition plugs 3 and fires the mixed gas in the combustion chamber 6. Then, the discharged current charges the capacitor 11 to a voltage limited by a Zener diode 10.
When the ignition current in the direction of arrow 3a is reduced to zero, the voltage held by the capacitor 11 is imposed on the ignition plugs 3. When combustion is normally effected in the combustion chamber 6 at the time, the ion current flows in the direction of arrow 3b.
A voltage at the point where the capacitor 11 is connected to a diode 12, i.e. a voltage output from the ion current sensing circuit 7 is a voltage at the inverting input of an inverting amplifier comprising of an operational amplifier 14 and a feedback resistor 15. When the operational amplifier 14 normally operates, the voltage becomes zero volt which is equal to a non-inverting input voltage. There are two types of cases in which the operational amplifier 14 does not normally operate, that is, they are a case in which a current flows in the direction of arrow 3a and a case in which an excessively large current flows in the direction of arrow 3b and an output from the operational amplifier 14 is saturated.
When a current flows in the direction of arrow 3a, a voltage output from the ion current sensing circuit 7 is used as a forward voltage (e.g. 0.7 V) of the diode 12, whereas when a large current flows in the direction of arrow 3b and the an output from the operational amplifier 14 is saturated, a diode 13 is conducted to thereby achieve a voltage reduced by an amount of the forward voltage. When the operational amplifier 14 normally operates, the ion current appears as a voltage drop across the feedback resistor 15 and is converted into a ground reference signal S4 as shown in FIG. 7. Note, in the signals S4 of FIG. 7, a ground reference signal from the internal combustion engine misfire sensing circuit 1a is represented by S4a and a ground reference signal from the internal combustion engine misfire sensing circuit 1b is represented by S4b. Subsequent signals S6 and S7 are also represented in the same manner.
As shown in FIG. 6, a leak current compensation feedback circuit 17 which is connected to the rear stage of the current/voltage conversion circuit 8 comprises a comparator 19 for comparing an output from the operational amplifier 14 with a threshold voltage of a reference voltage source 18, a capacitor 20 and a constant current charging/discharging circuit 21 of the capacitor 20. The leak current compensation feedback circuit 17 controls the output from the operational amplifier 14 so that it does not exceed the threshold voltage of the reference voltage source 18.
The waveform shaping circuit 9 comprises the comparator 19 for comparing the output from the operational amplifier 14 with the threshold voltage of the reference voltage source 18, a capacitor 22, a constant current charging/discharging circuit 23 of the capacitor 22 and a comparator 25 for comparing a voltage of the capacitor 22 with a threshold voltage of a reference voltage source 24. That is, the comparator 19 is shared by the current/voltage conversion circuit 8 and the waveform shaping circuit 9.
When the ion current is generated and the voltage output from the operational amplifier 14 is boosted and exceeds the threshold voltage of the reference voltage source 18, the capacitor 20 is charged and its voltage is boosted and a feedback current is increased. During the period in which the ion current is generated, a voltage output from the comparpator 19 is increased to a high level, whereby the capacitor 22 of the waveform shaping circuit 9 is charged and its voltage S6 is boosted as shown in S6 of FIG. 7. When the voltage S6 of the capacitor 22 exceeds the threshold voltage of the reference voltage source 24, a misfire sensing signal S7 as an output from the comparator 25 is made to a high level as shown in S7 of FIG. 7. The waveform shaping circuit 9 filtrates and outputs an ion current enduring for a predetermined period of time and removes an ion current caused by a leak current.
A four-cylinder engine has, for example, an ignition cycle of 5 ms at 1000 rpm, whereas an engine having the greater number of cylinders such as eight cylinders has a shorter ignition cycle of 2.5 ms at the same 1000 rpm. On the other hand, an ion current flows for about 2.5 ms after the occurrence of ignition. Therefore, when combustion intervals are close to each other as in the case of the eight-cylinder engine, since periods during which the ion current flows overlap, the known internal combustion engine misfire sensing circuit cannot sense the misfire of the eight-cylinder engine.
To cope with this problem, the known internal combustion engine misfire sensing circuit divides the cylinders into two groups to make combustion intervals coarse and employs the two sets of the internal combustion engine misfire sensing circuits 1a and 1b. That is, as shown in S7 of FIG. 7, the internal combustion engine misfire sensing circuit 1a senses the misfire of cylinders #1, #3, #5 and #7, whereas the internal combustion engine misfire sensing circuit 1b senses the misfire of cylinders #2, #4, #6 and #8.
The known internal combustion engine misfire sensing circuit has a problem that since periods during which an ion current flows overlap in, for example, the eight-cylinder engine, it cannot sense misfire.
To cope with this problem, the eight-cylinder engine needs two sets of internal combustion engine misfire sensing circuits, whereby a problem arises in that a plurality of signal lines are necessary to sense an ion current or misfire. Accordingly, miniaturization of an internal combustion engine misfire sensing circuit is prevented.